1. Field
The invention is directed toward electronic filters and methods of their use. Specifically, the invention is directed toward reflectionless electronic filters and methods of their use.
2. Background
Filters are ubiquitous components in virtually all electronic systems, from communications to radio astronomy, and arguably the fundamental principles of filter theory and optimization have been well known for the better part of a century. Nonetheless, practical filter design and implementation remains one of the most active fields of study in the electronics community today. However, reflectionless filters, in which the stop-band portion of the spectrum is absorbed rather than reflected back to the source, largely have been overlooked.
Standard reflective filters cause numerous problems in many applications, including: (1) sensitivity of mixer performance to out-of-band port terminations, (2) potential instability of integrated amplifiers with high out-of-band gain, (3) detrimental or unpredictable nonlinear effects resulting from reactive harmonic loading, (4) potential damage to high-power transmitters with large harmonic content, and (5) leakage, interference, or cross-talk resulting from trapped energy between the filter and another poorly matched component coupling into the enclosure cavity.
There are many practical situations in which the reactive termination presented by a conventional filter in its stop-band adversely affects the system performance. Mixers, for example, can be extremely sensitive to the out-of-band terminations present on any of their ports, which is precisely where a filter is most likely to be in many heterodyne applications. Wideband system designers have learned to work around this problem by routinely inserting fixed attenuators in the signal path near the mixer. Similarly, high gain amplifiers, though they may be unconditionally stable in a test fixture, can easily develop instability in a packaged environment where unintended feedback is combined with a reactive out-of-band termination on its input or output. Again, a filter is often used adjacent to the amplifier to better define the bandwidth of the system, and the stop-band impedance of the filter may need to be padded with attenuators to avoid causing stability problems.
One conventional approach to making a filter that is matched in both the pass- and stop-bands is to design a diplexer (or multiplexer) using two or more filters with complementary susceptance curves derived from singly loaded (reflective) prototypes, and terminating all but one of these filters with a matched load. This is a fairly complex procedure, requiring at least double the number of distinct elements as a conventional filter of the same order, and would be matched on only one of the two ports. An alternative design is to make a directional filter using two quadrature hybrids in a balanced configuration, or by using a directional coupling structure. However, quadrature hybrids with sufficient bandwidth can be difficult to design, and the intrinsically directional filter structures do not lend themselves to high-order implementations.
Therefore, it would be desirable to have a reflectionless filter that does not reflect signals in its stop-band back to the source. Further, it would be desirable to have a reflectionless filter that is well-matched at all frequencies, and on both ports.